The water gas shift reaction s widely used in synthesis gas related industries. Modern ammonia plants, hydrogen plants, as well as methanol plants use the shift reaction to improve overall plant efficiency. For instance, in ammonia plants, carbon monoxide leakage exiting shift reactors has a dramatic impact on plant economics. If the price of natural gas is $3 per MMBTU, each additional 0.1% CO leakage will cost $380,000 to $700.000 per year for a 1.500 TPD plant.
The shift reaction is exothermic and the carbon monoxide conversion is limited by thermodynamic equilibrium. Low exit carbon monoxide concentration can only be achieved at relatively low temperatures and/or high steam to gas ratios. Higher temperatures improve the reaction kinetics but can adversely affect catalyst life. Conventional technology uses staged, packed bed catalytic reactors with inter-bed cooling or shell-and-tube heat exchanger type of reactors with catalyst either in the tube side or the shell side such as described in U.S. Pat. No. 4,721,611. The reaction exotherm. e.g., 10.degree. C per 1% CO in feed for a typical steam to gas ratio, imposes a limitation on the maximum carbon monoxide conversion that can be achieved in a single conventional gas phase reactor.
U.S. Pat. No. 1,843,540 and Australian patent No. 9017/27 suggest a liquid phase CO shift process using water as the 11quid medium. Because the European Patent Application EP No. 0 163 532 A2 teaches the use of a solid catalyst to promote the shift reaction of CO and H.sub.2 O to CO.sub.2 and H.sub.2. Comparison Example 1 at Page 11 described the use of oxygenated non-aqueous solvents in which the catalyst is suspended while promoting the reaction between CO and H.sub.2 O. The solid catalyst is a Group VIII metal carbonyl compound chelated with a basic nitrogen-containing ring compound which may be suspended in an aqueous solution or bonded to a carrier material, shift catalysts require a reaction temperature of higher than 200.degree. C. and water has relatively high vapor pressures, these two patents are therefore limited to relatively high system pressure to avoid excessive vaporization of water. In fact, in an example, U.S. Pat. No. 1,843,540 (Column 3, Line 45) used 150 to 200 atmosphere for the shift reaction. Such a high system pressure requirement often makes the process impractical.
Two types of shift reactors are commonly used in the industry, the high temperature shift (HTS) and the low temperature shift (LTS) reactors. Normally, when the process stream is hot and contains excess steam a system consisting of HTS reactors followed by LTS reactors is used. HTS reactors, operated at around 370.degree.-590.degree. C. (700.degree.-1100.degree. F.), convert the bulk of the carbon monoxide in the feed gas, and LTS reactors, operated at around 200.degree.-260.degree. C. (390.degree.-500.degree. F:, polish the stream further. The state-of-art catalysts for HTS and LTS are Fe.sub.2 O.sub.3 /Cr.sub.2 O.sub.3 -based and o CuO/ZnO-based respectively. The CuO/ZnO ATS catalyst has a temperature limitation because it deactivates rapidly and becomes uneconomical at high temperatures, i.e. &gt;277.degree. C. (530.degree. F.). With the temperature limitations on carbon monoxide conversion and catalyst deactivation, it is important to control reactor temperature. Hot spots in the reactor should be minimized. This problem becomes more pronounced when carbon monoxide concentration in the gas stream is higher (because more reaction heat is to be generated). Another disadvantage associated with the conventional technology is that the disposal of spent HTS Fe.sub.2 O.sub.3 /Cr.sub.2 O.sub.3 catalyst has some environmental concerns, i.e. chromium.
The standard solution to cope with this temperature problem is using multiple reactors in series with both intra-bed and inter-bed quenching and/or exchanger cooling approach. In spite of these attempts, significant temperature gradients still exist in the packed catalyst beds and impose some temperature limitation on operations.
In the case of cool and dry feed gases, such as basic oxygen furnace (BOF) off-gas or certain coal gasifier effluent gas, these gases must first be heated to HTS reaction temperature (370.degree.-390.degree. C.) and steam added. This heating of a feed gas requires expensive heat exchange equipment and costly superheated steam, adding costs to the process.
Another solution to heating and saturating of cold dry gas feed streams is the use of heat exchangers and/or saturator-cooler vessels that exchange heat and water vapor from the shift converter effluent with the shift converter inlet. The cost for this equipment is a significant burden to the cost of shift conversion. Moreover, the use of multiple beds with inter-bed cooling makes it prohibitively expensive to shift gases containing high concentrations of carbon monoxide.